In recent years, there has been a renewed interest in the behavior of ion-containing polymers in fresh and high ionic strength aqueous media. These materials have a variety of useful properties, including the ability to expand its hydrodynamic volume in fresh water as the polymer concentration is diluted resulting in an increase in the solution viscosity. It is generally accepted that this expansion is due to the repulsion between like charges chemically bonded to the chain backbone (i.e., polyelectrolyte effect). However, if the influence that each charge has on each other is screened, than the chain will contract and the viscosity will correspondingly decrease. A very effective screening mode becomes operative through the addition of a soluble salt, such as sodium chloride. Therefore, these homogeneously charged polymers are not generally useful viscosifiers in high ionic strength medium.
Recently we have developed a novel class of ionomeric polymers in which cationic and anionic groups are chemically attached to the backbone chain. The hydrodynamic volume of these intramolecular complexes, i.e., polyampholytes, expand with the addition of soluble acids, bases, or salts. This is due primarily to the inability of the ionomeric monomer units to move freely into the bulk solution as found in classical polyelectrolytes. Thus, these complexes are more soluble in high ionic strength solutions than in fresh water and have a higher viscosity in the former than the latter solution. Moreover, an equimolar ratio of anionic and cationic groups are not required for these materials to function effectively.
We report the finding that specific intramolecular polymeric complexes, composed of neutral (acrylamide), cationic (methacrylamidopropyltrimethylammonium chloride), and anionic (sodium salt of styrene sulfonate) monomer units are capable of retaining its fresh water viscosification characteristics with the addition of a soluble salt. That is, the viscosity of these polymer solutions remains essentially unchanged with the addition of acid, base or salt. In qualitative terms, these polymers are polyampholytes with a relatively minor amount of dissociable and mobile charge which counterbalance (via charge screening mechanism) the previously detailed chain expansion. These viscosity characteristics are novel, since the general tendency of homogeneously charged macromolecules in these types of aqueous solutions shows a marked decrease in thickening efficiency.
These novel polymers can be useful in a variety of technologically interesting fluids as required in well control and workover fluids and in other systems where viscosity concentration relationships are required to be invarient with the addition of high levels of salt to the fresh water system.
Typical water soluble monomers incorporated into the terpolymers that are envisioned in the present invention are listed as follows:
Anionic: 2-acrylamido-2 methylpropane sulfonic acid, sodium styrene sulfonate, (meth) acrylic acid, 2-sulfoethylmethacrylate, and the like.
Cationic: methacrylamidopropyltrimethylammonium chloride, dimethyldiallylammonium chloride, diethyldiallylammonium chloride, 2-methacryloxy-2-ethyltrimethylammonium chloride, trimethylmethacryloxyethylammonium methosulfate, 2-acrylamido-2-methylpropyltrimethylammonium chloride, vinylbenzyltrimethylammonium chloride, and the like.
Nonionic: (N,N-dimethyl) acrylamide, hydroxyethyl (meth) acrylate, alkyl substituted acrylamides, (meth) acrylates, N-vinyllactanes (e.g., n-vinyl-2-pyrrolidone), and the like.
These monomers possess the appropriate water solubility for polymerization to take place.
Salamone et al., of the University of Lowell (Massachusetts), have investigated ampholytic polymers. They have studied the solution properties of divinylic cationic-anionic monomer pairs and also cationic-anionic monomer pairs with a neutral comonomer. This latter group of materials contains styrene as the neutral comonomer (J. Polym. Sci. Al, 18, 2983 [1980]), which can be incorporated into the ampholytic macromolecular structure through both solution or emulsion polymerization schemes. Apparently, other neutral vinylic monomers (i.e., acrylamide) were also polymerized (Gordon Research Conference--1981); but as of the present time, reports of this work have not been published in the scientific literature. However, in all of Salmone's work, detailed descriptions of his synthesis is reported. In all instances, the polymerization of the anionic-cationic monomeric species occurred via an "ion-pair comonomers that have no nonpolymerizable counterions present" (J. Polym. Sci. Letters, 15, 487 [1977]). The physical and chemical properties of these ion-pair comonomers are quite different than the individual ions (J. Polym. Sci. Letters 15, 487 [1977]).
Excess dissociable charges are not present within these polymeric materials.